1. Field of the Invention
The present invention relates to a flow diagnosis apparatus for a fuel vapor purge system.
2. Description of Related Art
In a previously proposed fuel vapor purge system, fuel vapor, which is generated in a fuel tank of a vehicle, is adsorbed to an adsorbent material received in a canister to limit leakage of the fuel vapor from the fuel tank to the surrounding atmosphere. Then, the fuel vapor, which is adsorbed to the adsorbent material in the canister, is desorbed from the adsorbent material and is purged into an air intake system of an internal combustion engine, which supplies fresh air to combustion chambers of the internal combustion engine. The purging of the fuel vapor is accomplished by action of a negative pressure in the air intake system upon opening of a purge control valve provided in a purge passage, which communicates between the canister and the air intake system and forms a fuel vapor system in cooperation with the fuel tank and the canister. Here, it is necessary to sense a leakage of the fuel vapor from the fuel vapor purge system to the atmosphere in an early stage to limit a prolonged leakage of the fuel vapor from the fuel vapor purge system to the atmosphere.
Japanese Unexamined Patent Publication No. 2004-300997A discloses a technique for addressing the above need. According to this technique, a leak check module is connected to a canister of a fuel vapor purge system. The leak check module has a pressure sensor, a vacuum pump, a reference pressure sensing arrangement and a passage change valve. First of all, the vacuum pump is driven to introduce the negative pressure into the reference pressure sensing arrangement. Then, the reference pressure is sensed with the pressure sensor. Thereafter, the negative pressure introducing passage of the vacuum pump is changed by driving the passage change valve to introduce the negative pressure into the fuel vapor system, which includes the fuel tank. Then, the pressure in the fuel vapor system is sensed with the pressure sensor. The pressure in the fuel vapor system is compared with the reference pressure to determine whether a leak exists in the fuel vapor system with the high accuracy.
In the fuel vapor purge system, the fuel vapor passage conducts fuel vapor from the fuel tank and purges the fuel vapor into the air intake system of the internal combustion engine. When the fuel vapor passage is clogged with a foreign object (e.g., debris, dusts or the like), the fuel vapor in the fuel tank cannot be purged into the air intake system. The inventor of the present invention has conducted various experiments about a flow diagnosis that determines whether the fuel vapor passage is clogged based on behavior (e.g., a decreasing speed) of the pressure in the fuel vapor system, which is sensed with the pressure sensor at the time of introducing the negative pressure from the air intake system, into the fuel vapor system by opening the purge control valve while the engine is running. The inventor of the present invention has encountered the following problems through the experiments.
In the case of Japanese Unexamined Patent Publication No. 2004-300997A where the leak check module is connected to the canister of the fuel vapor purge system, the pressure sensor is connected to the canister provided in the fuel vapor passage, which extends from the fuel tank to the air intake system of the engine. In this structure, in a case (see FIG. 4) where the clogging occurs in a portion of the fuel vapor passage located on the air intake system side of the pressure sensor (canister), the pressure does not decrease substantially or decreases at the reduced speed at the location of the pressure sensor. In view of this, it is possible to sense the clogging of the fuel vapor passage by monitoring the behavior of the sensed pressure, which is sensed with the pressure sensor during the period of introducing the negative pressure into the fuel vapor system.
However, the pressure at the sensing location of the pressure sensor decreases rapidly at the time of introducing the negative pressure from the air intake system into the fuel vapor passage in a case (see FIGS. 7A and 7B) where the clogging occurs in another portion of the fuel vapor passage located on the fuel tank side of the pressure sensor (canister) as well as in a case (see FIGS. 6A and 6B) where a fuel vapor inlet of the fuel vapor passage is closed by a shutoff valve in the full level of the fuel tank. Therefore, the difference in the behavior (e.g., the decreasing speed) of the sensed pressure, which is sensed with the pressure sensor during the period of introducing the negative pressure into the fuel vapor system, becomes small between these two cases. Therefore, even when the behavior of the pressure, which is sensed with the pressure sensor during the period of introducing the negative pressure into the fuel vapor system, is monitored, it is difficult to distinguish between the case where the clogging occurs in the other portion of the fuel vapor passage located on the fuel tank side of the pressure sensor and the case where the fuel level in the fuel tank is the full level. Thus, it is difficult to accurately determine whether the other portion of the fuel vapor passage located on the fuel tank side of the pressure sensor is clogged with the high accuracy.